The disclosure relates in general to the field of communication on field buses that are implemented as serial asynchronous data buses. The disclosure is especially applicable to field bus systems, such as Profibus, Foundation Field bus, CAN (Controller Area Network), but also to Ethernet and others. A field bus connects field devices, such as sensors and actuators, in a single system for communicating with a computer. Communication takes place via bus-specific telegrams that are specified in a bus-specific protocol. Usually a field bus system comprises at least one master field device (Master) for controlling the system, as well as a plurality of slave field devices (Slaves).
To increase the availability of field bus systems it is known to provide line redundancy, master redundancy and/or slave redundancy in any desired combination.
Line redundancy means that the field bus is realized with at least two parallel, redundant conductors or lines A and B. In the case of line redundancy, the bus lines for the application are thus implemented transparently in duplicate. This means that the failure of a bus line can be tolerated without any restriction of functionality.
Master redundancy is usually implemented by the field bus system being built with a primary master and a backup master which are both connected to both lines A and B, provided that the field bus is also line-redundant. This implies a complete duplication of the master. The redundant master is transparent from the user's point of view, is thus not apparent in normal operation, and regardless of any existing line redundancy present. Existing slaves can be used without modification and are operated by one or the other master. Switching between the masters takes place without interruption and without loss of process data or re-initialization of the bus. This will ensure that outputs of the slaves do not decay and the operation can be continued without interruption or malfunction. A defective master can be replaced safely at any time. A newly inserted assembly is automatically calibrated and is available again after a short period of time. There is always exactly one master active, the redundant master is used as a backup in the event that the primary master fails.
According to the PROFIBUS standard, slave redundancy means e.g. that each slave has interface modules for all redundant structures, so for example, two interface modules for a line-redundant field bus with two lines A and B. In addition, the Slave has a redundancy communication channel (RedCom). Redundant Slaves therefore comprise a primary communication interface and a backup communication interface, each with its own, different bus address assigned. The master always communicates with the primary communication interface and only to a limited extent with the backup communication interface. For example, in PROFIBUS only one slave interface may be active at any time, having regard to operational data transmission. In the event of a failure of only one PROFIBUS line or of only one connection, the communication with the other slave interface is resumed without any effect on the process outputs. Both communication interfaces perform a self-diagnosis and send their diagnostic information to the master, the diagnostic information of the backup interface being included in the diagnosis of the primary interface and being sent via this.
The master has a redundancy extension for monitoring the communication capacity of the redundant slave. If the redundancy extension of the master detects a fault in the communication with the primary slave, it sends a command to the backup communication interface, so that the system is switched over to the latter. In addition, a redundant slave can monitor its internal components, not related to the communication with the field bus, for certain error conditions and can switch over from the primary to the backup communication channel if an internal error is detected. The monitoring of interference in the communication always takes place via the master.
During operation, only one of the communication interfaces of the slave is active at any time, be it the primary or the backup communication interface, and only one interface is configured. When switching from one interface to the other the new interface can receive all of the necessary data on the redundancy communication channel Red-Com, or it must wait for the first data exchange with the master field device following the switchover. This can lead to undesired delay, because this type of redundancy requires two completely separate bus interfaces. These and more details of the Profibus slave redundancy are described in the specification “Slave Redundancy”, version 1.2, November 2004, published by Profibus User Organization (www.profibuscentre.com.au/ducs/slave-redundancy.pdf).
In the prior art it is also known to equip serial bus systems which lack system-integrated redundancy means with an upstream connectable redundancy extension. A suitable device for this purpose is, for example, the Profibus Redundancy Link Module RLM01 from the ABB AG, Mannheim, Del. (http://www.abb.de/product/seitp334/289cb36695cb150dc12571c6002e1c0b.aspx), which is also described in EP 0 990 330 B1. This redundancy extension is a series connected device which can be inserted between a non-redundant master or slave field device and a line-redundant serial bus with two lines A and B. The module regenerates the signal shape and amplitude of all received data and monitors all lines for activity and error conditions. The initially incoming data in a line-redundant serial bus over line A or line B with a correct telegram start are forwarded by the redundancy extension module via an interface M to the master or slave field device. In the case of a data frame containing an erroneous telegram start sent on line A, the system is switched over to the still available redundant line B, and vice versa. Data blocks sent on the bus from the field devices via the redundancy extension module are passed through in parallel to the two lines A and B. A disadvantage of this known series connected device is first of all that, a separate, unwieldy device must additionally be installed in the field-bus device, one which also requires its own power supply as well as the manual setting of the bit rate.
In addition, the series connectable device introduces an additional delay into the data transfer, which is similar in its effect to an additional propagation delay. This is because the switching device requires a certain processing time for receiving and interpreting the data, in order to select the correct receiving line. For this purpose the reception of at least one byte is necessary which, for example according to the PROFIBUS standard, introduces a latency of at least 11 bit periods. Only once the redundancy extension module has decided which line A or B is selected as the receiving line will the data be forwarded to the actual field devices. For communication between the master and slave, which comprises sending at least one command from the master to the slave and sending back a response from the slave to the master, there will therefore be a delay of twice the minimum necessary time for processing a data frame, in the case of PROFIBUS 2×11 bits. Depending on the transmission rate, which in PROFIBUS can be e.g. in the range of 1.5 kBaud to 12 MBaud, this will result in delay times of the order of several microseconds. The delay of 11 bit periods per receive process has the same order of magnitude as the normal response time of the station, i.e. the response time is de facto doubled. Such delays can be highly relevant in time-critical process control tasks.
The ABB system has the additional disadvantage that it only examines the beginning of a data frame for communication errors, and does not take account of faults which may occur after the first character of a message. On the other hand, in this system a more comprehensive analysis of the received data blocks would lead to a still larger delay, which would also have a negative effect. Indeed, if a data block, so for example, a telegram, were to be completely examined for errors before the line selection is made, then it must be received completely before its onward transmission. The delay introduced would correspond to the length of the telegram, which in many cases would no longer be within the tolerance allowed by the protocol.
U.S. Pat. No. 6,594,227 describes a system for controlling the communication between different stations over a line-redundant Ethernet bus according to the TCP/IP protocol using a special type of management of the MAC addresses of a receiver, a transmitter and a network adapter.
US-A-2010/0290,339 also describes an Ethernet network in which in this case redundancy is created by the connections being provided in duplicate with each one being alternately addressable with the same network address.
DE 10 2010 015 455 A1 discloses a method for the redundant connection of a field device to a field bus, in which the network interfaces are also implemented with redundancy. Here also, the devices are controlled using a special management of the MAC addresses.